1. Field of the Invention
The present invention relates to a method and a system for encrypted communications using a multi-valued modulation scheme. The invention more particularly relates to a method for synchronization of a running key between a transmitting node and a receiving node in an encryption communication system using a multi-valued modulation scheme representing Yuen quantum cryptography, and to detection of a shift of the synchronization of the running key and re-synchronization of the running key between the transmitting and receiving nodes.
2. Description of the Related Art
Yuen quantum cryptography is also referred to as quantum cryptography for optical communications. In Yuen quantum cryptography, quantum fluctuations (quantum shot noise) of light are spread by modulation so that an eavesdropper cannot properly receive an optical signal and that data cannot be deciphered even by infinite computational power. Thus, in recent years, it has been proposed that Yuen quantum cryptography be applied to a shared key encryption scheme. The shared key encryption scheme is designed so that a binary optical signal carrying data to be transmitted in a binary code is regarded as one set (which is called a basis), and that an M number of the bases are prepared, and that a basis used to transmit data among the bases is randomly determined based on the pseudo-random number that is in accordance with an encryption key. Practically, a multi-valued optical signal is designed such that an inter-signal distance is small enough to prevent data identification due to quantum fluctuations. Thus, the eavesdropper cannot decipher data information from a received signal.
Optical modulation/demodulation devices used in the legitimate transmitting node and the legitimate receiving node operate to communicate data while changing the M number of the binary bases in accordance with common pseudo-random numbers. Thus, the legitimate receiving node can read data by detecting a binary signal having a large inter-signal distance. As a result, an error due to quantum fluctuations can be ignored, whereas it is makes it possible to provide appropriate communications.
The encryption based on the abovementioned optical modulation scheme is called Yuen quantum cryptography based on Yuen-2000 cryptographic communication protocol (abbreviated to Y-00 Protocol). Northwestern University where P. Kumar, H. Yuen, et al. are enrolled has disclosed an optical phase modulation technique, which is a communication scheme that allows for the Y-00 Protocol, in Non-Patent Document 1 (G. A. Barbosa, E. Corndorf, P. Kumar, H. P. Yuen, “Secure communication using mesoscopic coherent state,” Phys. Rev. Lett. Vol-90, 227901, (2003)). In addition, a group of Tamagawa University has disclosed an optical intensity modulation technique in Non-Patent Document 2 (O. Hirota, K. Kato, M. Sohma, T. Usuda, K. Harasawa, “Quantum stream cipher based on optical communication” SPIE Proc. on Quantum Communications and Quantum Imaging vol-5551, (2004)).
In a quantum encryption scheme in optical communications using a multi-valued modulation scheme representing Yuen quantum cryptography which is an optical intensity modulation scheme, a basis of a signal to be transmitted is determined in accordance with a running key that is generated from an encryption key. In order to receive the signal, it is necessary that a threshold value that is used to discriminate the signal should be changed so as to obtain an optimal level of the discrimination in accordance with a running key. In this case, when the synchronization of the running key that is used for encryption and decryption and that is shared between a transmitting node and a receiving node is shifted, the receiving node cannot properly decrypt the received signal.
The shift of the synchronization of the running key may be caused by: a failure or the like in a transmission system such as a fluctuation of the intensity of light for transmission and a fluctuation of the wavelength of light for transmission due to a change in temperature in a transmission device; a failure or the like in a receiving system such as a timing detection error; a failure or the like in a network such as disconnection of an optical fiber and a failure in a router; an inoperative status such as power breakdown in either of the transmitting node or the receiving node; and the like.
In order to reinitiate encrypted communications in the state in which the synchronization of the running key is shifted due to at least one of the abovementioned failures or the like, it is necessary that the transmitting node and the receiving node share the position of the running key and communications be initiated in the state in which the position of the running key in the transmitting node is the same as that in the receiving node. In encrypted communications using a multi-valued modulation scheme based on Yuen quantum cryptography, if a third party does not have the encryption key or uses an incorrect key, he cannot obtain proper information. Thus, even when a key is shifted as much as conventional mathematical encryptions, encrypted data cannot be deciphered by temporarily storing received data and sequentially applying the key. Therefore, it is necessary that the synchronization of the running key be established without reduction in encryption strength during encrypted communications, and that the position of the running key, which is used when re-synchronization is performed, be properly shared when a shift of the synchronization occurs.